Conventional ocean altimetry methods are based on air- or space-borne monostatic radars. These methods are mostly limited to nadir-looking instruments with a comparatively narrow swath, and therefore they are not able to provide high spatial-temporal sampling, necessary to map properly ocean mesoscale features, unless deployed in ad-hoc constellations.
The wide swath ocean altimeter has been envisaged as a potential solution to increase spatiotemporal sampling; however it is a very complex and costly solution. See W. J. Emery, D. G. Baldwin, D. K. Matthews, “Sampling the Mesoscale Ocean Surface Currents With Various Satellite Altimeter Configurations”, IEEE Transactions on Geoscience and Remote Sensing, Vol. 42, No. 4, April 2004, 795.
In this context, since 1993 the European Space Agency and later European industry have been working on the idea to make use of GNSS (Global navigation Satellite System) signals reflected from the ocean's surface in order to perform altimetry. The technique, designated as “PARIS” (Passive Reflectometry and Interferometry System), has been investigated and experimentally proven by ESA, NASA and many other space and non-space related research organizations. PARIS is a very wide swath altimeter, capable of reaching a swath of 1000 km or more, depending on orbital altitude, as it picks up ocean-reflected (and direct) signals from several GNSS satellites, up to an average of 12 tracks, and up to a maximum number of 16-20 tracks, when Galileo will be available.
For a detailed description of the PARIS technique, see the following documents:                M. Martin-Neira, “A Passive Reflectometry and Interferometry system (PARIS): Application to Ocean Altimetry”, ESA Journal, 1993;        U.S. Pat. No. 5,546,087 to M. Martin-Neira; and        G. A. Hajj, C. Zuffada, “Theoretical Description of a Bistatic System for Ocean Altimetry Using the GPS Signal”, Radio Science, Vol 38, No 5, October 2003.        
A PARIS altimeter essentially comprises an up-looking antenna, a down-looking antenna and signal processing means. The up-looking antenna is directed to the outer space, and receives signals emitted by one ore more “opportunity” satellite emitters, e.g. navigation signals emitted by GNSS satellites. The down-looking antenna is directed to the Earth, and receives replicas of these same signals reflected by the Earth surface. The signal processing means compute the distance of the apparatus from a specular reflection point of the signal on the Earth surface by cross-correlating the signals received by the up-looking and the down-looking antennas. From this distance, and assuming precise knowledge of the apparatus position, it is possible to determine the altitude of said specular reflection point with respect to a reference geoid. U.S. Pat. No. 5,546,087 discloses that the down-looking antenna should preferably by a highly directive phased array with multiple steerable receiving lobes directed towards the reflection points of the GNSS signals over the ocean, but it does not provide any specific teaching about the up-looking antenna. All PARIS experiments and proposals for flight versions of this altimeter up to now have assumed a low-gain up-looking antenna. In practice, this implies the need of generating code replicas on board, limiting the operation of the system to open access codes.